Abstract Project 2 of the Sedentary Time and Aging Research (STAR) Program will investigate how 3 month changes in standing time, brief sit-to-stand transitions and physical activity (PA) breaks, will impact biomarkers of healthy aging, and physical, emotional and cognitive functioning compared to healthy living attention controls. This 4-arm randomized controlled trial (RCT) will occur in the `real world' with postmenopausal women (N=592) who are not physically active, do not stand up frequently and spend at least 8 hours sitting per day, i.e. most older adults. The daily behavior targets in each condition of Project 2 (increase standing by 2 hours, increase sit-to-stand transitions by 30 per day, & increase PA breaks by 12) reflect the treatment conditions explored in Project 1. To date, there have been no RCT studies with health outcomes in older adults. Our multiple pilot studies, however, demonstrate the feasibility and acceptability of delivering intervention materials (including standing desks, trackers & ActivPAL feedback) to older adults and evaluating behavior change objectively with thigh-worn inclinometers. The interventions significantly reduced sitting time (up to 2 hours a day) and significantly increased sit-to-stand transitions (up to 40 per day). Increases in PA were minimal in the standing conditions, compared to our PA focused RCT in 307 older adults which increased short PA bouts. One pre-post pilot found that reductions in sitting time were significantly related to improved gait speed and decreased depressive symptoms. Project 2 will assess the impact of the 3 interventions to interrupt sitting compared to the attention control condition at 6 and 12 weeks. We will recruit 592 overweight, sedentary, postmenopausal women who will be randomized to one of the 4 conditions. We will employ tools such as standing desks, wrist-worn device alerts and condition-specific, tailored ActivPAL feedback. The primary outcomes are glucose regulation and blood pressure. Secondary outcomes include mitochondrial functioning (measured by muscle mitochondrial respiratory capacity), and physical, emotional and cognitive functioning. Exploratory outcomes include psychosocial and environmental mediators and moderators of behavior change. We will also explore the moderating effect of age on the intervention outcomes. Project 2 compliments the other projects by assessing 3 month intermediary outcomes. Project 2 will help validate the novel computational algorithms optimized in Project 1 and applied to Project 3. In addition, the Biostatistics Core will investigate the response patterns in Projects 1 and 3 and map them to Project 2. Project 2 will strengthen the public health evidence of how to interrupt sitting time in older adults, provide specific information on interrupting sitting as an alternative to longer PA bouts, and explore a range of targeted outcomes related to healthy aging.